1. Field of the Invention
The invention relates to circuits used to turn on and off solid-state power switches by applying drive signals to their gates, and more particularly to a novel approach for simplifying the driver circuit.
2. Prior Art Drive Circuitry
Present day driver circuits for solid-state switches such as bi-polar transistors and SCR's, utilize a separate power supply to supply drive power to the gate. If the power supply is external to the driver, it will generally be operated at relatively high voltage and the current will be transmitted at high frequency in order to keep transformer size to a minimum. Depending on the size of the power switch to be driven and the switching rate, the supply current may be high, and the power supply has to be sized accordingly to meet its needs. The power supply and its switching circuits must be electrically isolated from the transistor gate and the signal source, because of possible high common mode voltages from the transistor which result in transmitted noise. In the best known driver circuit mechanizations, only about 1500 volts isolation is typically available, using isolating transformers. Noise transmission from the power transistor is often problematic, necessitating elaborate schemes for suppression of these disruptive signal elements. The resulting drive circuits, are therefore complex and expensive, typically costing hundreds of dollars each or even thousands where high reliability components are used as in military applications. In a typical 3-phase power converter, as many as twelve or twentyfour driver boards may be used, costing thousands of dollars. Thus, there exists a need for a simple driver circuit that does not need a separate power supply; that has high voltage isolation and is relatively inexpensive.